The present invention relates generally to photographic projection printing. In particular, it relates to the use of an optical system employing a diffusing surface which reduces the visual effects of artifacts.
In making photographic copies or enlargements of prints and/or transparencies it is desired to do so in a manner which facilitates the minimization of visually observable imperfections that might be projected to the print. Moreover, in making such enlargements it is often desirable to selectively increase and decrease the amount of light in different areas of the screen. For instance, it is desirable to increase light in the shadows and reduce the amount of light in the highlights. This approach produces prints which overcome or compensate for certain imbalances of tones in the transparency or print which is to be copied. Heretofore, it has been known to selectively shade different portions of a projected image, such as by using a mask. This technique is generally referred to as dodging. Conversely, selectively increasing the intensity of the exposure to preselected areas is a technique known as burning-in.
In electronic imaging or enlargement, instead of using a conventional light as a source of illumination, use is usually made of a flying spot scanner. In a flying spot scanner an electron beam is caused to move over the surface of a cathode ray tube. This is done in a preselected pattern to produce predetermined illumination areas having selected degrees of illumination. The brightness of these areas can be regulated by monitoring the light emerging from the transparency to be copied. This can be done by, for example, a phototube. In this manner, an automatic and simultaneous burning and dodging of various parts of the negative can be done in accordance with the local density of the negative.
In typical electronic enlarging applications, a raster pattern generator defines a pattern having rather coarse or large areas of illumination. For example, it may define a checkerboard pattern having areas of alternating brightness and darkness. In practice, these areas of illumination, when projected onto the transparency, sometimes define on the print rather sharp steps in brightness along the edges thereof. These sharp steps in brightness define a fringing type of optical artifact. Fringing produces unacceptable prints. Other kinds of artifacts are caused by imperfections in the illumination system, for instance, scratches or dirt and debris on the cathode ray tube. Artifacts are undesirable.
One known attempt to minimize artifacts, particularly of the fringing kind, is to smudge the sharp steps in brightness along the edges of the areas. Smudging or blurring can be accomplished by defocusing the cathode ray tube. However, there still exists an edge function on the illumination which while diminished tends nevertheless to be observable. Another approach for diminishing artifacts is described generally in commonly assigned U.S. Pat. No. 4,457,618. In that patent there is described an aperture mask placed in front of a flying spot illumination source. The aperture mask is a graded transmission density member having specific characteristics by which fringing effects or other artifacts for that matter are made to appear less noticeable. While the aperture mask is successful it nonetheless requires use of a special graded density filter.